“But they could make sure they sleep in a room that is completely dark or they could wear eye-masks to let night-time melatonin rise and take Tamoxifen right before going to bed, that would be the easiest way to see if it works.”

Some studies have suggested that melatonin may improve cancer treatments.

It has been suggested that the hormone calms down cancer cells, which are working on overdrive, to slow down the processes needed to develop resistance.

Dr Hill wants to move the research into human breast tissue.

Evening tech

He did add a warning about using technology, which tends to produce blue light wavelengths that disrupt melatonin production, in the run-up to bedtime.

“If you get seven hours sleep at night, but use an iPad or computer or watch TV then those blue wavelengths are suppressing melatonin production for anywhere from an hour to an hour and half.

“So instead of seven hours of melatonin, you’re getting six or five and a half.”

Dr Samuel Godfrey, from the charity Cancer Research UK, said: “The link between developing drug-resistant breast cancer and sleeping in dim light is an intriguing one, but it’s important that we remember this link was found after studying a small number of rats implanted with breast cancer cells, not humans.

“More research in people is needed before we know whether the same mechanisms are at work in people.”